I really do think you are worrying a little too much.
The lady friend has no more right than any friend or relative he has stopping over a few nights a week.
It may change if she actually leaves her own home and moves in, but that can quite easily be dealt with by creating a tenancy agreement, even if it is a peppercorn rent. Even without the rent it is highly unlikely she would acquire any rights. The bungalow is not your dad's to leave in a will, nor does he have the sort of tenancy that can be 'inherited'.
Is there a possibility of her selling her own home, or giving up her tenancy? You might remind your dad of his responsibility to inform the council if his entitlement to the sole occupancy council tax rebate is affected.